1. Field of the Invention
The invention concerns an electro-optic voltage sensor for determining the voltage on an electric line, comprising a voltage divider adapted to be connected to the electric line and to ground and having two insulative compartments separated by an intermediate electrode connected to a Pockels cell, the intermediate electrodes supplying to the Pockels cell a reduced voltage derived from the voltage supplied by the electric line to the voltage divider.
2. Description of the Prior Art
A sensor of the above kind is used in distribution networks to determine the voltage on electric lines needed for "Metering" transmission and "Protecting" the network.
The Pockels cell comprises an electro-optic crystal, for example an oxide of bismuth and germanium or of bismuth and silicon. The Pockels effect is the name of the phenomenon whereby birefringence of the crystal is caused by application of an electric voltage between two faces of the crystal covered with a conductive deposit.
The Pockels cell also comprises an input optical fiber connected to a light-emitting diode emitting an incident light beam through a polarizer and a phase-shifter plate. On passing through the electro-optic crystal the polarization of the incident light beam varies. An analyzer on the path of the transmitted light beam converts the polarization variation at the output of the crystal into a variation of luminous intensity conveyed by an output optical fiber to a photodiode detector in a remote electronic unit.
The polarization variation depends on the voltage applied between the conductive faces of the electro-optic crystal.
It can be shown that in one particular mode of operation of the Pockels cell the ratio of the instantaneous luminous power received by the photodiode to the continuous power emitted by the light-emitting diode depends on the applied voltage U in the form of a sine of the product k*U where k is an electro-optic coupling coefficient characteristic of the crystal of the Pockels cell used.
The Pockels cell can determine an applied voltage U up to a typical value 10,000 volts, usually called the quarter-wave voltage of the electro-optic crystal.
Beyond this limiting value, the evolution with time of the instantaneous luminous power received relative to the evolution with time of the applied voltage U is no longer monotonous.
A first solution to this problem is to double the incident light beam in order to produce another trigonometric variation of the luminous power.
This has two major drawbacks, however. First, the necessity to use a long (currently 25 cm) electro-optic crystal to withstand the voltage constitutes a technical limitation and a very high additional cost. Second, the bandwidth of the electro-optic sensor is considerably limited and is incompatible with the measurement of fast transient voltage signals like those corresponding to the specifications of the "Protection" channel.
A second solution is to associate the Pockels cell with a voltage divider. The latter generally comprises a capacitive column formed of identical individual capacitors consisting, for example, of sheets of aluminum serving as armatures separated by insulative films of paper or polypropylene, the whole being immersed in an insulative liquid, for example an oil-based liquid.
An intermediate electrode electrically connected to one of the conductive faces of the electro-optic crystal of the Pockels cell separates the capacitive column into two insulative compartments. A total voltage applied to the column generates a reduced voltage at the Pockels cell, the ratio between the voltages being determined by the capacitances of the two compartments of the column.
To determine the tension on the electric line the Pockels cell is associated with a capacitive column disposed inside and along the entire length of an insulator supporting the electric line. The length of the insulator filled with a pressurized gas is chosen according to the voltage on the electric line so that the capacitance of the two compartments divides the line voltage sufficiently for the reduced voltage to be less than the quarter-wave voltage of the electro-optic crystal of the Pockels cell used.
An electro-optic voltage sensor of the above kind gives rise to two problems, however.
First, in the event of an internal short-circuit, due for example to failure of the insulation of the capacitive column, the resulting increase in temperature and pressure generally causes the insulator to explode, whether it is made of a vitreous material such as porcelain or a composite material.
Second, the capacitive column is in practice vertical in the insulator, which can suffer mechanically caused variations in its dimensions due to differential crushing of the individual capacitors at the bottom and at the top of the column, which phenomenon can be even more marked if the oil-based insulative liquid is replaced with a gas. Thermally caused variations in its dimensions can result from differential expansion of the individual capacitors due to a temperature gradient between the bottom and the top of the column. The consequence of such dimensional variations is measurement errors incompatible with the "Metering" channel of the electro-optic sensor.
The aim of the invention is therefore to propose an electro-optic voltage sensor comprising a Pockels cell associated with a voltage divider that offers increased safety vis a vis risks of short-circuit and which confers on the sensor high reliability and high measurement accuracy.